Fatigue-resistant threaded component for a tubular threaded joint

ABSTRACT

A fatigue-resistant threaded element for a threaded tubular connection. A portion of threads of the threaded element have a helical groove opening into the threaded crest. A groove is formed in all or a portion of the threads of one or both end zones, namely the first engaged thread zone and the last engaged thread zone, and it is optionally formed in the threads of the medial thread zone. The groove reduces stiffness of threads carrying a groove in the end zone or zones compared with the stiffness of the threads in the medial thread zone. The threaded tubular connection obtained after making up the threaded element with a mating threaded element is much more resistant to cyclic tensile stress and/or axial compressive stresses and/or flexion stresses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a male or female threaded element of athreaded tubular connection that is particularly capable of resistingboth static and cyclic stresses, and to a threaded tubular connectionthat is particularly capable of resisting both static and cyclicstresses.

2. Discussion of the Background

Threaded tubular connections comprise a male threaded element at the endof a first pipe, generally a great-length pipe, and a threaded femaleelement at the end of a second pipe, which may be a great-length tube ora coupling.

They are used to constitute casing strings or production strings ordrilling strings for hydrocarbon wells or for similar wells such asgeothermal wells.

Specifications 5B and 5CT from the American Petroleum Institute (API)define threaded connections between casing pipes or between productionpipes with tapered threads.

Premium threaded tubular connections, which have sealing characteristicsdespite a wide variety of stresses, have been described, for example, inEuropean patent EP 488 912 and in United States patent U.S. Pat. No.5,687,999.

Such threaded connections can employ straight or tapered threads withone or two threaded portions.

Until very recently, casing or production strings essentially had to becapable of resisting different combinations of static stresses (axialtension, axial compression, planar flexion, internal pressure orexternal pressure) despite their limited thickness resulting from thenecessity, in order to run a deep well, of nesting strings withdifferent diameters one inside the other.

In contrast, drillpipes, which are only used to drill wells, aredesigned to resist substantial cyclic stresses but are not subjected toconstraints on their internal space, as a single drillpipe string of agiven diameter is lowered at a given time.

When drillpipe strings are in operation, if not strictly limited, cyclicstresses lead to fatigue cracking initiated at the thread root,generally on the side of the load flanks, more particularly at the lastengaged threads of the threaded elements.

In the remainder of the present document, the term “first threads” isused to designate the threads which, in a longitudinal cross-sectionpassing through the axis of the threaded element, are located on theside of the front end of the threaded element. Consequently, the lastthreads are those situated at the other end of the threading.

The term “engaged threads” means firstly, the threads of the threadedelements of a threaded tubular connection in the made up state whichaxially transfer the load from one threaded element to a mating threadedelement, whether the threads are perfect (full height) or imperfect(incomplete or partial height, for example run-out or run-in threads).When the threaded connection is subjected to axial tensile stresses,which is generally the case, the engaged threads are those for which theload flanks are in contact.

By extension, the term “engaged threads” of an isolated threaded elementmeans threads intended to transfer the load to the corresponding threadsof a mating threaded element when these two threaded elements areconnected to constitute a threaded tubular connection.

The position of the engaged threads of a threaded element is known fromthe design of the threaded element as this is a necessary datum forpredicting the strength of the resulting threaded connection. Theposition of the last or first engaged threads can thus be completelydefined on a threaded element intended to produce a threaded tubularconnection.

The problem of fatigue behaviour, however, no longer applies solely todrilling strings but also for strings for running certain hydrocarbonwells wherein the threaded tubular connections that constitute suchstrings must be capable of supporting both high static stresses andcyclic stresses.

Such stress behavioural requirements are encountered in offshore stringsconnecting the seabed to offshore hydrocarbon production platforms.

Such strings, known as “risers” in the English-speaking language of theskilled person are subjected to cyclic stresses caused in particular bycurrents that cause the string to vibrate, by the swell, by the tide andany possible displacement of the platforms themselves.

Such stress behaviour is also encountered with onshore wells, inparticular when lowering rotating pipes for cementing wells in the veryfrequent case of wells that deviate from the vertical and bend.

The prior art concerning threaded tubular or non-tubular connections (ofthe bolted type, for example) proposes means for improving the fatiguebehaviour of threaded connections subjected to axial tensile loads thatmay vary in a cyclic fashion.

U.S. Pat. No. 3,933,074 describes a nut for a bolted connection whereinthe internal threading is interrupted at the first engaged threads by aplurality of axial channels regularly disposed on the periphery of thethreading to displace the maximum axial tensile stress transfer zonebetween the nut and screw from the first female engaged thread to themiddle of the axial length of the nut.

Such channels, the length of which can be half the length of thethreading and the depth of which can be as high as 80% of the threadheight, increase the flexibility of the first engaged threads but reduceby about 20% the bearing surface of the threads in the zone in whichthey are formed, which is a disadvantage when high resistance to staticstresses is required and the threaded tubular connection has to besealed to the interior and exterior of the pipes.

Further, solutions for the bolts in which the nuts bear on the side ofthe first threads against the screw head (on the side of the last screwthreads) are not necessarily directly applicable to threaded tubularconnections.

Further, International patent applications WO 00/14441 and WO 00/14442describe threaded connections that comprise a groove in the threads toreduce the stiffness of the thread structure with the aim of reducingthe makeup torque. Those documents disclose no mode of the groove itselfthat can improve the resistance of the threaded connections to cyclicstresses.

SUMMARY OF THE INVENTION

The present invention seeks to produce a male or female threaded elementfor threaded tubular connections, which is particularly resistant bothto:

-   -   a) static stresses, in particular axial tension, axial        compression, flexion, torsion, internal or external pressure,        jump-out during makeup, either alone or in combination (for        example tension+internal pressure);    -   b) cyclic tension-compression stresses or flexion stresses by        reducing load transfer in the first and last engaged thread        zones and by minimising stress concentration factors (SCF) in        those zones.

The present invention also seeks to provide a threaded element that canbe produced with all types of threadings: tapered, straight,straight-tapered combinations, in one or more steps, with radiallyinterfering threads and/or with flanks in contact; threads in contactvia their 2 flanks with the corresponding flanks of the mating threadare, for example, of the type known as “rugged thread” described in EP 0454 147, of the axial interference fit type described in WO00/14441 orof the variable width wedge type as described in United States reissuedpatent U.S. Re-30 647.

The present invention also seeks to provide a threaded element that caneasily be produced and inspected.

The threaded element of the invention must be capable of being used tomake threaded tubular connections intended for hydrocarbon productionstrings, casing strings or for risers, or intended for similar uses.

The invention also seeks to provide threaded tubular connections thathave a particularly good seal, in particular to gas, even under cyclicstresses.

The invention also seeks to provide a threaded element that can beemployed to constitute drillpipe strings.

The present invention also seeks to provide a threaded tubularconnection in which one or two threaded elements have been modified toresist cyclic stresses.

The male or female threaded element of the invention is produced at theend of a pipe and comprises an external male threading or an internalfemale threading depending on whether the threaded element is a malethreaded element or a female threaded element.

It is intended to be connected to a mating threaded element (i.e.,female if the threaded element under consideration is male and viceversa) to constitute a threaded tubular connection that is resistant tocyclic stresses.

The threading is constituted by at least one threaded portion. When thethreading comprises a plurality of threaded portions, these can bespaced from each other axially and/or radially, for example in steppedthreaded portions.

The or each threaded portion of the threading comprises three zones ofsubstantially identical length, starting from the front end of thethreaded element: a zone termed first engaged thread zone, a zone termedmedial thread zone and a zone termed last engaged thread zone, thedefinition of the first and last engaged threads corresponding to thatindicated above in the prior art.

Certain of these zones can comprise partial height threads such asrun-in or run-out threads.

A helical groove is produced substantially radially in the threads of atleast a fraction of the axial length of at least one threaded portion,the groove opening into the thread crest, defining either side of thegroove a load half-thread and a stabbing half-thread respectively on theload flank side and on the stabbing flank side. The groove can, however,only partially open into the thread crest.

In accordance with one characteristic of the invention, in each threadedportion where it is formed, the groove is formed over all or a portionof the threads of one or two end zones, namely the first engaged threadzone and the last engaged thread zone and it can optionally be producedin the threads of the medial thread zone; the geometricalcharacteristics of the groove are such that they reduce the stiffness ofthe threads with grooves in the end zone or zones with respect to thestiffness of the threads in the medial thread zones.

The stiffness of the grooved threads is determined by the flexion and/orshear ability of the stressed half-threads, which are generally the loadhalf-threads taking into account the axial tensile stresses to whichthreaded tubular connections are generally subjected; the same inventiveconcept can, however, be adapted to stabbing half-threads when these arestressed or are also stressed, for example in threaded tubularconnections operating in compression.

The stiffness of a thread is defined as the coefficient ofproportionality between the axial load transferred by the thread underconsideration to the corresponding thread of a mating threaded elementof a threaded tubular connection, and the axial deformation undergone bythe thread under consideration.

In a threaded tubular connection, the groove of the invention reduces,with respect to a similar threaded tubular connection with non groovedthreads, the axial load transfer between the threads of the zone orzones of threads in which the groove is formed and the correspondingthreads of the mating threaded element by redistributing the overallaxial load between the different engaged threads of the threading; thissignificantly improves the resistance of the threaded tubular connectionto dynamic stresses, in particular flexion, superimposed on staticstresses in axial tension.

Since in a threaded tubular connection, the first engaged threads of athreaded element co-operate with the last engaged threads of the matingthreaded element, disposing a groove either at the level of the firstengaged threads of each of the two threaded elements or at the level ofthe last engaged threads or both at the same time on one only or boththreaded elements produces the same technical effect of equalising theload transfer per thread in the first and last threads of the threadedportions in which the groove is produced with respect to the loadtransfer per thread in the medial thread zone.

The groove can also be produced in all or a portion of the threads ofthe medial thread zone provided that it further reduces the stiffness ofthe threads with the groove in the end zone or zones over the stiffnessof the threads in the medial thread zones.

It is then possible to improve or even optimise all the threads of thethreaded portion in which a groove is formed and of the correspondingthreaded portion of the mating threaded element of a threaded tubularconnection.

It should be noted that the groove of the invention can also reducestresses on the threads of the first or second engaged thread zonesgenerated by errors in the pitch between the male and female threads ofa threaded tubular connection, the pitch error resulting from tolerancesin the threading manufacture.

It can also limit dangerous overpressures caused by greasing andlubricating the threads on makeup.

Preferably, the groove is produced in the threads of the first engagedthread zone and optionally in the threads of the medial thread zone andit does not affect the threads in the last engaged thread zone whichremain solid, to reduce the stiffness of the threads in the firstengaged thread zone compared with the stiffness of the threads in themedial thread zone.

The inventors have shown in this case that the groove of the inventionwould reduce the maximum value of the stress concentration coefficient(SCF) of the wall section at the mating thread root, the SCF being arelative dimension obtained by relating the maximum stress at thelocation under consideration to the stress in the corresponding pipebody. The groove of the invention thus reduces the maximum stress in thewall at the thread root of the last engaged thread zone of the matingthreading, which wall is in this thread zone subjected to the overallaxial tensile load on the threaded tubular connection and thus reducesthe risk of fatigue crack initiation at that location.

Thus, the threaded elements of a threaded tubular connection can be usedwith large cyclic variations in load without modifying the axial tensionperformance.

Preferably, the groove is obtained using a forming tool on the threadsof the threaded portion under consideration. This means that its form isdefined by the profile of the forming tool and that its depth, measuredfrom the thread crest to the groove bottom, is defined by thepenetration depth of the forming tool in the threads.

Preferably, the stiffness of the threads increases steadily because ofthe groove from the end engaged thread of the end zone or zones with agroove going towards the medial thread zones.

The end engaged thread of an end zone is the first engaged thread whenthe groove is produced in the threads of the first engaged thread zoneand the last engaged thread when the groove is produced in the threadsof the last engaged thread zone. Thus, both the first and last engagedthreads are involved when the groove is formed in both the threads ofthe first engaged thread zone and the last engaged thread zone.

Advantageously, the depth of the groove reduces, preferably regularly,from the end engaged thread of the end zone or zones with a groove goingtowards the medial thread zone.

Alternatively or supplementarilly, the groove has a helical pitch thatis different from that of the threaded portion in which it is formed.

Preferably, the envelope of the groove bottoms is a conical surface thatis coaxial with the axis of the threaded element.

In a variation, the envelope of the groove bottoms is a surface ofrevolution that is coaxial with the axis of the threaded element andwith a non rectilinear generatrix such as a toric surface, a parabolicsurface or a hyperboloid or a composite surface of a plurality ofsurfaces of revolution connected to each other end to end.

In one or more of these variations, when the groove is produced in thefirst engaged thread zone, the slope of the generatrix of the envelopeof the corresponding groove bottoms is preferably greater than the slopeof the threaded portion in which the groove is implanted, whether thelatter slope is positive (tapered threadings) or zero (straightthreadings). It is preferably lower than that of the threaded portionwhen the groove is produced in the last engaged thread zone.

Advantageously, to allow engagement of the male threaded element in thefemale threaded element under good conditions, the groove does not openonto the stabbing flanks when the threads are generally trapezoidal inshape.

Advantageously again, the threaded element can comprise an abutment withan abutting surface that is constituted by the front end of the threadedelement and which is under axial compression.

Since the groove is formed at the level of the first engaged threads andreduces the axial stiffness thereof, it can advantageously accumulate anabsolute deformation of the axial compression in the abutment at the endof makeup of the threaded connection constituted with a threaded tubularconnection.

When the threading is separated from the front end of the threadedelement by a relatively short or non-existent lip, the groove canincrease the axial length that is effectively deformed by compressioncompared with a similar prior art threaded element and can thusaccumulate a higher absolute deformation in the abutment. When seekingan optimum seal, there is frequently advantage in having a short lipwhen a sealing surface is produced at the peripheral end of the lip.

In a variation, the threaded element can comprise a first abutment thesurface of which is constituted by the front end of the element underconsideration and a second abutment disposed in a co-operating manner ona made up threaded tubular connection with an abutment at the front endof a mating threaded element. In this case, the presence of groove(s) onthe threaded elements can advantageously allow both of the abutments ofthe threaded element under consideration to be in bearing contactagainst two corresponding abutments of the mating threaded element.

Such a bearing pair is normally rendered difficult in prior art threadedtubular connections unless the two abutments are very accurately andexpensively produced one with respect to the other, or the lips areextended, with discouraging effects. The increase in the effectivelength of axial compression of the abutments by the grooves means thatthis double bearing can be more readily produced than on a prior artthreaded tubular connection with two sets of abutments.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages will become apparent from the embodiments describedbelow and from the accompanying drawings.

FIG. 1 is a diagrammatic representation of an axial half cross-sectionof a female threaded element of a threaded tubular connection of theinvention.

FIG. 2 shows the threading of the threaded element of FIG. 1 with thethreads during the course of machining.

FIG. 3 shows a detail of the first engaged threads of the femalethreaded element of FIG. 1.

FIG. 4 is a diagrammatic representation of an axial half cross-sectionof a male threaded element of the invention adapted to the femalethreaded element of FIG. 1.

FIG. 5 shows the threading of the male threaded element of FIG. 4 withthe threads during the course of machining.

FIG. 6 shows a detail of the first engaged threads of the male threadedelement of FIG. 4.

FIG. 7 shows an axial half cross-section of a threaded tubularconnection of the invention obtained after makeup of the threadedelements of FIGS. 1 to 4.

FIG. 8 shows an axial half cross-section of a variation of the threadedtubular connection of the invention.

FIG. 9 shows an axial half cross-section of a further variation of thethreaded tubular connection of the invention.

FIG. 10 shows a detail of the first engaged threads of a variation ofthe female threaded element of the invention.

FIG. 11 shows a detail of the first engaged threads of a variation ofthe male threaded element of the invention.

FIG. 12 diagrammatically shows the change in load transfer between thethreads of a standard threaded tubular connection and of a threadedtubular connection of the invention.

FIG. 13 diagrammatically shows, in the same manner as FIG. 12, thechange in the stress concentration coefficient at the thread root forthe male threading and for the female threading.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a female threaded element 2 disposed at the end of a pipe102.

Pipe 102 can be a great-length pipe, i.e., about 10 m or more in length,or a coupling a few tens of centimeters in length, only half shown inFIG. 1. In the first case, the female threaded element 2 can produce an“integral” threaded connection; in the second case, a threaded andcoupled connection is produced.

The inside of the female threaded element 2 comprises, from its frontend 10, a female threading 4 composed of a single threaded portion, asealing surface 6 and an abutment 8.

The female sealing surface 6 is a conical surface inclined at 20° to theaxis X1X1 of threaded element 2.

Abutment 8 is a substantially transverse surface and more particularlyit is slightly convexly conical and forms an internal shoulder onthreaded element 2.

The female threading 4 is tapered with a peak half angle of 1.79°(taper=6.25%) with trapezoidal threads 12.

Threading 4 comprises a first engaged thread zone 32 constituted by thefirst six threads of the front end side 10 of the threaded element, alast engaged thread zone 36 constituted by the six threads from thepenultimate female thread and a medial thread zone 34 comprising sixthreads between zones 32 and 36.

The last thread of the threading is not designed to be an engaged thread(see FIG. 7).

The number of threads in zones 32 and 36 of the first and last engagedthreads corresponds to ⅓ of the total number of engaged threads.

As shown in detail in FIG. 3, the female threads 12 generally comprise athread crest 18, a thread root 20, a stabbing flank 16 turned towardsthe front end 10 of the threaded element and a load flank 14 on theopposite flank.

A helical groove 22 is machined in threads 12 using a forming tool 42that is independent of the cutting of threads 12.

This forming tool 42 is in the form of an inverted rounded V defined byan angle of 35° between the arms of the V and a rounded top with aradius of 0.4 mm.

Tool 42 is positioned to cut the threads substantially radially from thethread crest in the shape of a V with a rounded bottom and leaving twohalf-threads either side of the tool, a load half-thread 24 on the loadflank 14 side and a stabbing half-thread 26 on the stabbing flank 16side, without cutting into the thread flanks.

The forming tool 42 is displaced as shown in FIG. 2 from the firstthread 12.1 following a helix with a pitch p identical to the pitch ofthe female threading 4, the base of the tool bearing on a conicalsurface the generatrix of which is seen at 44. This conical surface hasa taper that is twice that of threading 4 (angle of 3.58° betweengeneratrix 44 and the axis of the threaded element) so that the depth ofgroove 22 steadily reduces from the first engaged thread 12.1 andbecomes zero at the eleventh thread 12.11 and beyond to the last thread.

Groove 22 can reduce the stiffness of the threads in first engagedthread zone 32 compared with the stiffness of the threads in medialthread zone 34.

Once made up, since the threads 12 are subjected to contact pressure onthe load flanks 14, their stiffness is determined by the ability of theload half thread 24 to flex, in particular by dint of its geometry.

This geometry can be characterized by the inclination of the load flank14 and the flank of groove 28 compared with the axis of the threadedelement, by distance d2 between the load flank 14 and centre O₂ of therounded zone of the groove bottom and by the distance d4 between thepoint O₂ and the envelope of the thread roots. Since the pitch of thehelix of groove 22 is the same as that of threads 12, distance d2 variesonly very slightly from one thread to the next.

Because of the greater slope of generatrix 44 with respect to that ofthe threading, distance d4 increases continuously from the first thread12.1 so that d4.1<d4.2<d4.3 and so on.

This means that the stiffness of the female threads 12 is a minimum atthe threads in first engaged thread zone 32 and a maximum at thegroove-free threads of the last engaged thread zone 36 and of medialthread zone 34; the stiffness of the female threads 12 of first engagedthread zone 32 is also less than that of the threads with shallowgrooves in medial thread zone 34.

The stiffness of the threads increases steadily with the reduction inthe groove depth from the first engaged thread 12.1 (end thread) goingtowards the medial thread zone 34.

The distance d4.1 is slightly greater than the radius R2 of the bottomof the groove so that at no time does the groove bottom extend beyondthe envelope of the thread roots with no groove.

However, it is possible to produce, to no great disadvantage, a groovethat cuts the envelope of the roots of the first threads (for exampled4.1=0) provided that the critical cross section of the threaded element2 (which in service supports all of the axial load on threaded element2) is situated at the last thread which has no groove.

In contrast, when a groove is formed in the last engaged thread zone,the bottom of the groove formed must not extend outside the volumeincluded within the envelope of the bottoms 20 and that of the threadcrests 18 if the service performance of the threaded tubular connectionincluding the threaded element 2 is not to deteriorate.

FIG. 4 shows a male threaded element 1 disposed at the end of agreat-length pipe 101.

The exterior of threaded element 1 comprises, from its front end 7forming an abutment, a sealing surface 5 and a male threading 3.

Abutment 7 is a slightly conical concave surface intended to co-operatewith the abutment 8 on female threaded element 2.

Sealing surface 5 is a conical surface inclined at 20° to the axis X1X1of threaded element 1 and co-operates with female sealing surface 6.

Male threading 3 is composed of a single threaded portion; it is taperedand adapted to co-operate with female threading 4.

It comprises eighteen trapezoidal engaged threads 11, the last eightthreads with reference numeral 37 being incomplete in height (threadstermed “vanishing” or “run-out” threads.

The first six threads constitute the first engaged element zone 31, thefirst thread being chamfered on the front end to facilitate engagement.

The last six threads, all run-out threads, form last engaged thread zone35.

The six intermediate threads form medial thread zone 33.

As was the case for the female threads, male threads 11 comprise athread crest 19, a thread root 17, a stabbing flank 15 turned towardsthe front end 7 of the threaded element and a load flank 13 on theopposite flank (see FIG. 6).

A helical groove 21 is machined in threads 11 using a forming tool 41(similar to that used to machine groove 22 in female threads 12); thegroove is machined independently of the cutting of threads 11.

The forming tool 41 cuts the threads substantially radially from thethread crest leaving two half-threads either side of it, a loadhalf-thread 23 on the load flank side and a stabbing half-thread 25 onthe stabbing flank side, without cutting into the thread flanks.

The forming tool 41 is displaced as shown in FIG. 5 along a helix withpitch p that is identical to the pitch of the male threading 3, the baseof the tool bearing on a conical surface with generatrix 43.

This conical surface has a taper that is twice that of threading 3(i.e., an angle of 3.58° between generatrix 43 and the axis of threadedelement 1) so that the depth of groove 21 steadily reduces from thefirst engaged thread 11.1 to become zero at the 10^(th) thread 11.10.

Groove 21 can reduce the stiffness of the threads in first engaged zone31 with respect to the stiffness of the medial thread zone 33.

As was the case for the female threads, the stiffness of the malethreads is determined by the geometry of the load half-thread 23 and inparticular by the inclination of the load flank 13 and the groove flank27 with respect to the axis of the threaded element, by the distance d1between the load flank 13 and the centre O₁ of the rounded zone of thebottom of the groove and by the distance d3 between the point O₁ and theenvelope line of the thread roots.

The pitch of the helix of groove 21 is the same as that of threads 11,the distance d1 varying only slightly from one thread to the next.

Because of the greater slope of the generatrix 43 with respect to thatof the threading, distance d3 increases continuously from the firstthread 11.1 so that we have: d3.1<d3.2<d3.3 and so on.

This means that the stiffness of the male threads 11 is a minimum at thethreads of the first engaged thread zone 31 and a maximum at thegroove-free threads in the last engaged thread zone 35 and the medialthread zone 33; the stiffness of the male threads 11 of first engagedthread zone 31 is also lower than that of the threads with shallowgrooves in the medial thread zone 33.

The stiffness of male threads 11 increases steadily with the reductionin depth of the groove from the first engaged thread 11.1 to the tenththread in medial thread zone 35.

The distance d3.1 is slightly higher than the radius R1 of the groovebottom (0.4 mm) so that the groove bottom never extends beyond theenvelope of the thread roots but, as was the case of the female threads,a groove could be formed that cut the envelope of the roots of the firstthreads.

However, a groove must not be formed in the last engaged thread zonewherein the bottom extends beyond the volume included within theenvelope of the bottoms 17 and that of the thread crests 19, thecritical cross-section of the male threaded element being located at thelevel of the last engaged thread.

FIG. 7 shows the threaded tubular connection 100 constituted by makingup the threaded elements 1 and 2 of FIGS. 1 and 4 to the specifiedmakeup torque.

The conical male sealing surface 5 radially interferes with the conicalfemale sealing surface 6 and the male abutment 7 is forced to bearagainst the female abutment 8.

In reaction to the axial compressive stresses between abutments, theload flanks 13, 14 of the male and female threads bear against eachother and develop contact pressures.

Further, the female thread crests 18 radially interfere with the rootsof the male threads 17 while there remains a clearance between the malethread crests 19 and the female thread roots 20.

FIG. 7 shows the engaged threads and the position of grooves 21 and 22in the first engaged thread zones 31, 32 and in a fraction of the medialthread zone 33, 35.

FIG. 12 shows the axial load transfer F_(A) per thread between theengaged male and female thread load flanks on threaded tubularconnections stressed by axial tension under a load such that the body ofpipe 101 is stressed to 80% of the yield strength of the material (80%PBYS).

Curve B relates to the threaded tubular connection of FIG. 7 of theinvention, while curve A relates to a similar standard threaded tubularconnection but without a groove.

Curve A (standard threaded tubular connection) has a dished appearance;load transfer peaks at the first and last engaged threads; the medialthread zones 33, 34 cannot therefore be used to their full load transfercapacity.

Curve B (threaded tubular connection of FIG. 7) shows a much moreuniform load transfer thanks to grooves 21, 22 which reduce thestiffness of the first engaged threads.

This curve shows that the threaded tubular connection of FIG. 7demonstrates excellent behaviour both as regards static stresses(mechanical strength, seal) and as regards dynamic stresses (resistanceto initiation of fatigue cracking).

A similar effect can be obtained with a groove produced in the lastengaged threads or at both the first and last engaged threads. In thecase of a groove produced in the last engaged thread zone, the slope ofthe generatrix of the envelope of the groove bottoms should, of course,be lower in this zone than that of the threaded portion to obtain thereducing effect on the thread stiffness.

The external loads on the threaded elements of a threaded tubularconnection and the stresses resulting from makeup result in a stressfield that may have a maximum in the junction radius at the thread rootbetween the load flank and the thread root.

It is convenient to determine a stress concentration coefficient (SCF)for each thread at this location by expressing it in terms of the stressin the pipe body 101 and in particular to define the stressconcentration coefficient in accordance with International standard ISO13628-7CD1:SCF=(σ_(principal thread)(T _(max))−σ_(principal thread)(T_(min))/σ_(pipe body)(T _(max))−σ_(pipe body)(T _(min)))

T_(min) and T_(max) are the loads corresponding to a stress in the pipebody 101 in axial tension, for example 0 and 80% of the yield strength;

σ_(principal thread) is the highest of the three principal stresses onan elemental cube of material taking into account both the stresses frommakeup and those to which the threaded tubular connection is subjected(axial tension+alternating flexion, for example);

σ_(pipe body) is the stress on the pipe body 101 such that thedenominator of the SCF in the selected example is 80% of the effectiveyield strength of the pipe under consideration.

FIG. 13 shows values of SCF on the side of the male threaded element(curves A1 and B1) and on the side of the female threaded element (curveB2), curve A1 relating to a standard threaded connection and curves B1and B2 relating to a threaded connection of the invention (FIG. 7).

The importance of the groove on the fatigue behaviour is apparent incurves A1 and B1 of FIG. 13: compared with a prior art threaded tubularconnection (curve A1), groove 22 (curve B1) reduces the SCF peak at themale last thread zone to increase the value of the SCF peak in the zoneof the first male threads; but this peak is not very deleterious asregards fatigue as the wall of the male threaded element at the firstmale thread zone is not very stressed in axial tension while the wall ofthe male threaded element at the level of the last male thread zone musttolerate the overall axial tension load on the threaded element.

Groove 21 acts in a similar manner on the shape of the SCF curveregarding the female threads, the wall of the female threaded element atthe first female threads being in compression because of abutments 7, 8;curve B2 of FIG. 13 is to be compared with a curve with an appearancesimilar to curve A2 in the same figure.

The above reasoning can also be directly applied in the case of combinedexternal stresses: static axial tension and internal static pressure andcyclic flexion, as an example. It can also be applied to cases in whichthe threaded elements are stressed in axial compression by adapting thedisposition of the groove (stabbing half-threads stressed rather thanload half-threads).

FIG. 8 shows a variation in a threaded tubular connection for riserscomprising, in addition to a set of internal sealing surfaces 5, 6 as isthe case for FIG. 7, a set of external sealing surfaces 45, 46 toprevent external or internal fluid from ingress.

In addition to the internal abutments 7, 8 shown in FIG. 7 (principalabutments), the threaded connection of FIG. 8 comprises externalabutments constituted by the front end surface 10 of the female threadedelement and by a corresponding annular surface 47 on the male threadedelement.

The male and female threadings 3, 4 are entirely similar to those ofFIG. 7, with a groove with a decreasing depth affecting the threads ofthe first female thread to the tenth female thread and of the first malethread to the ninth male thread and procuring the same technical effectof reducing the stiffness of the thread and reducing the maximum SCFvalues.

The grooves also permit greater functional flexibility of the externaland internal abutments.

The deep groove produced in the first male threads and the low stiffnessthereof increases the effective length over which the male lip 9 iscompressed at the end of makeup: lip 9 is then compressed over a lengththat is longer than its length and for the same admissible stress level,it is possible to make up the threaded tubular connection still furtherand give more energy to the sealing surfaces 5, 6.

Such a technical effect may already be advantageous for threaded tubularconnections with a single set of abutments of the type shown in FIG. 7but it is even more advantageous in the case of threaded tubularconnections with two sets of abutments as shown in FIG. 8.

It is difficult to synchronise the action of two sets of abutments apartfrom machining these two sets extremely accurately and thereforeexpensively.

The large deformation capacity of male lip 9 and female lip 50 enablesnot only the principal abutments (internal in the present case) but alsothe auxiliary abutments (external in the present case) to function inabutment in all mating scenarios between the male threaded element andthe female threaded element, even when the distance between the two maleabutments is a maximum and that between the two female abutments is aminimum.

It is possible to obtain a similar technical effect by extending thelips 9, 50 but this would reduce the compactness of the threaded tubularconnection, which is not desired, and would ruin its sealing power: iflips 9, 50 were too flexible, an insufficient contact pressure would beinduced between sealing surfaces 5, 6, 45, 46.

FIG. 9 shows a further variation in the threaded connection of theinvention which comprises, as in U.S. Pat. No. 5,687,999, male andfemale tapered threadings, each with two threaded portions 203, 203′,204, 204′ that are radially and axially apart from each other andseparated by a central set of abutments 207, 208.

Each tapered threaded portion comprises a fraction of run-in threads211, 211′, 214, 214′ in which the envelope of the thread roots istruncated parallel to the axis of the threaded element and a fraction ofrun-out threads 212, 212′, 213, 213′ in which the thread crests aretruncated parallel to the axis of the threaded element.

Each threaded portion comprises nine threads, all engaged, delimitingfirst engaged thread zones 231, 231′, 232, 232′, last engaged threadzones 235, 235′, 236, 236′ and medial thread zones 233, 233′, 234, 234′,each zone comprising three threads.

As shown in FIG. 9, a groove is machined in the first four male threadsand the first four female threads of each threaded portion to a depththat reduces from the first engaged thread to the fourth engaged thread.

The technical effect of the grooves is the same over each threadedportion as in the case of the threaded connection of FIG. 7 withthreadings in a single threaded portion and reduces the value of the SCFat the root of the last engaged threads of each threaded portion.

FIG. 10 shows a variation in the female threaded element of FIGS. 1 to3.

In FIG. 10, a groove is machined into the threads with a pitch p″ thatis smaller than the pitch p of the threading so that the distance d2 tothe load flank increases from the first female engaged thread:d2.1<d2.2<d2.3.

At least over the first threads, the base of the forming tool 42 formachining the groove is displaced over a conical surface with the sametaper as the female threading so that the depth of the groove issubstantially constant over these first threads.

The distance d2.1 is such that the groove does not open onto the loadflank.

After machining the groove over three thread pitches, the tool isretracted, its base following a curve 44 which is, for example, an arcof a circle or a hyperbola and describes a toric surface or a hyperbolaof revolution, so that that the groove does not open onto the stabbingflank, which could deleteriously affect the mutual engagement of themale threadings and the female threadings.

The slope of the curve 44 beyond the third thread is greater than thatof the threading to obtain the desired retraction.

The same technical effect of stiffness reduction of the first engagedthreads is obtained with the female threaded element of FIG. 11 as withthose of FIGS. 1 to 3.

FIG. 11 shows a variation in the male threaded element of FIGS. 4 to 6in which a groove is machined as shown in FIG. 11 with a pitch p′ thatis smaller than the pitch p of the threading and to an identical depthover the first threads.

The distance d1 from the groove to the load flank increases from thefirst male engaged thread: d1.1<d1.2<d1.3.

The base of the tool 41 for machining the groove, and as a result thetool and its point, follow a composite surface of revolution: the baseis firstly displaced along a conical surface with the same taper as thatof the male threading, then it follows a toric surface or a hyperboloidof revolution described by the generatrix 43 the slope of which isgreater than that of the threading.

As was the case for FIG. 10, this can achieve a reduction in thestiffness of the first engaged threads without cutting into the stabbingflank of the male threads.

Many other variations and embodiments that have not been described inthe present document are included in the scope of the invention asclaimed.

By way of non limiting example, the groove can be produced in anythreading type (straight, tapered, straight-tapered) with any threadtype (radially interfering, “rugged thread” of the type described in EP0 454 147, variable width wedges, with axial interference fit) or anygeneral thread form (trapezoidal, rounded triangular); the grooves mayhave a U profile; the groove bottom may describe a toric surface or ahyperbola of revolution from the first thread, the groove may beproduced both with a variable pitch and variable depth.

The thread flanks, in particular the load flank and/or the stabbingflank, can also be convexly bowed so as to control the contactcharacteristics (location, pressure) between the corresponding flanks inspite of the variation in the stresses when in operation.

The junction zones between the flanks and thread roots can also have aplurality of portions with different radii of curvature to minimise theSCF.

The peripheral surface of the threaded element opposite to that in whichthe threading is formed can also comprise a waist in the form of agroove produced at the level of the threading to reduce the residualwall thickness under the threading of the first engaged threads.

Particularly in the case of “rugged thread”, wedge or axial interferencefit threads in which the two thread flanks can be subjected to contactpressures of varying intensity, it is possible to exploit the technicaleffect of a groove with a pitch equal to that of the threading but witha variable depth: such a groove reduces the stiffness of the threadsboth on the load flank side and on the stabbing flank side and improvesthe fatigue behaviour of the threaded tubular connection in cyclictension, in cyclic compression, in tension-compression or in alternatingflexion at the same time.

1. A threaded element of a threaded tubular connection comprising: athreading with at least one threaded portion, each threaded portioncomprising, starting from an end of the at least one threaded portionlocated on a side of a front end of the threaded element, a firstengaged thread zone with at least three threads of substantiallyconstant width, a medial thread zone having a plurality of threads withsaid substantially constant width, and a last engaged thread zone withat least three threads of said substantially constant width; a helicalgroove formed substantially radially to at least partially open at athread crest, wherein the groove is formed in said at least threethreads of one or both of the first engaged thread zone and the lastengaged thread zone, and geometrical characteristics of the groovereducing a stiffness of at least the at least three threads ofsubstantially constant width and having the groove in one or both of thefirst and last engaged thread zones compared with a stiffness of thethreads in the medial thread zone having said substantially constantwidth such that the stiffness of the threads in said medial zone withsaid substantially constant width is greater than the stiffness of theat least three threads of substantially constant width in the one orboth of the first and last engaged thread zones and having the groove.2. A threaded element according to claim 1, wherein the groove is formedin the threads of the first engaged thread zone and optionally in thethreads of the medial thread zone, the threads of the last engagedthread zone remaining solid.
 3. A threaded element according to claim 1,wherein the groove has a profile obtained by penetration to a givendepth of a forming tool into the threads of the at least one threadedportion.
 4. A threaded element according to claim 1, wherein the groovehas a profile of a V with a rounded bottom.
 5. A threaded elementaccording to claim 1, wherein the groove is formed in the last engagedthread zone, and a bottom of said groove is located in a volume of thelast engaged thread zone located between an envelope of the thread rootsand an envelop of the thread crests.
 6. A threaded element according toclaim 1, wherein the geometrical characteristics of the groove are suchthat the stiffness of the threads having said groove increases steadilyfrom an end engaged thread of one or both of the first and last engagedthread zones towards the medial thread zone.
 7. A threaded elementaccording to claim 6, wherein the groove has a depth that reducesregularly from the end engaged thread of one or both of the first andlast engaged thread zones towards the medial thread zone.
 8. A threadedelement according to claim 6, wherein the groove has a helical pitchthat is different from that of the threads of the threaded portion inwhich the groove is formed.
 9. A threaded element according to claim 7,wherein an envelope of groove bottoms of said groove with said depththat reduces regularly is a conical surface coaxial with the threadedelement.
 10. A threaded element according to claim 7, wherein anenvelope of groove bottoms of said groove with said depth that reducesregularly is a surface of revolution coaxial with the threaded elementand with a non-rectilinear generatrix.
 11. A threaded element accordingto claim 9, wherein the groove is formed in the first engaged threadzone such that a slope of the generatrix of the envelope of the groovebottoms is greater than a slope of the threaded portion in which thegroove is formed.
 12. A threaded element according to claim 1, whereinthe threads have a generally trapezoidal form and the groove does notopen onto a stabbing flank of the threads.
 13. A threaded elementaccording to claim 1, wherein the threading comprises at least twothreaded portions, and wherein the groove is produced in each of the atleast two threaded portions.
 14. A threaded element according to claim1, further comprising a single abutment, an abutment surface of whichcomprises a front end of the threaded element.
 15. A threaded elementaccording to claim 1, further comprising: an external abutment, anabutment surface of which comprises a front end of the threaded element,and an internal abutment, wherein both of the external and internalabutments are configured to bear against corresponding abutments of amating threaded element.
 16. A threaded element according to claim 1,further comprising at least one sealing surface configured for radialinterference with a sealing surface on a mating threaded element.
 17. Athreaded tubular connection comprising: a male threaded element at anend of a first pipe and a female threaded element at an end of a secondpipe, at least one of the two threaded elements being a threaded elementaccording to claim
 1. 18. A threaded element according to claim 1,wherein the groove is not formed in the threads of the medial threadzone.
 19. A threaded element according to claim 18, wherein the groovein the threads of one or both of the first and last engaged thread zoneshas a depth that reduces regularly towards the medial thread zone.
 20. Athreaded element according to claim 19, wherein the groove in thethreads of the first engaged thread zone has a depth that reducesregularly towards the medial thread zone.
 21. A threaded elementaccording to claim 20, wherein the groove in the threads of the lastengaged thread zone has a depth that reduces regularly towards themedial thread zone.
 22. A threaded element according to claim 1, whereinthe groove is formed in the threads of the first and last engaged threadzones and not in the threads of the medial thread zone.
 23. A threadedelement according to claim 1, wherein the groove is formed in thethreads of the first engaged thread zone and not in the threads of themedial thread zone.
 24. A threaded element according to claim 23,wherein the groove in the threads of the first engaged thread zone has adepth that reduces regularly towards the medial thread zone.
 25. Athreaded element according to claim 1, wherein the groove is formed inthe threads of the last engaged thread zone and not in the threads ofthe medial thread zone.
 26. A threaded element according to claim 25,wherein the groove in the threads of the last engaged thread zone has adepth that reduces regularly towards the medial thread zone.
 27. Athreaded tubular connection comprising: a male threaded element at anend of a first pipe and a female threaded element at an end of a secondpipe, each of the male and female threaded elements being a threadedelement according to claim
 1. 28. A threaded tubular connectionaccording to claim 27, wherein the groove is formed in the threads ofthe first engaged thread zone of the male element and not in the threadsof the last engaged thread zone of the female element, wherein said lastengaged thread zone of the female element engages with said firstengaged thread zone of the male element when said threaded tubularconnection is made-up.
 29. A threaded tubular connection according toclaim 28, wherein the groove is formed in the threads of the firstengaged thread zone of the female element and not in the threads of thelast engaged thread zone of the male element.
 30. A threaded tubularconnection according to claim 29, wherein the groove is not formed inthe threads of the medial thread zone of the male and female elements.31. A threaded tubular connection according to claim 27, wherein thegroove is formed in the threads of the first engaged thread zone of thefemale element and not in the threads of the last engaged thread zone ofthe male element, wherein said last engaged thread zone of the maleelement engages with said first engaged thread zone of the femaleelement when said threaded tubular connection is made-up.
 32. A threadedelement according to claim 1, wherein the threaded element is male andsaid threading is an external male threading.
 33. A threaded elementaccording to claim 1, wherein the threaded element is female and saidthreading is an internal female threading.
 34. A threaded elementaccording to claim 1, wherein the groove is formed in the threads of themedial thread zone having said substantially constant width.
 35. Athreaded element according to claim 34, wherein the medial thread zonehas at least three threads having said substantially constant width. 36.A threaded element according to claim 1, wherein the groove is formed inthe threads of the first engaged thread zone and in the threads of thelast engaged thread zone.
 37. A threaded element according to claim 36,wherein the groove is formed in a first thread of the first engagedthread zone, wherein said threaded element is free of any thread in aportion between said threading and said front end of the threadedelement, and wherein said first thread has said substantially constantwidth.
 38. A threaded element according to claim 36, wherein the grooveis formed in a last thread of the last engaged thread zone, wherein saidthreaded element is free of any thread in a portion between saidthreading and a back end of the threaded element, and wherein said lastthread has said substantially constant width.
 39. A threaded elementaccording to claim 1, wherein the groove is formed in the threads of thefirst engaged thread zone and not in the threads of the last engagedthread zone.
 40. A threaded element according to claim 39, wherein thegroove is formed in a first thread of the first engaged thread zone,wherein said threaded element is free of any thread in a portion betweensaid threading and said front end of the threaded element, and whereinsaid first thread has said substantially constant width.
 41. A threadedelement according to claim 40, wherein said at least three threads ofsaid first engaged thread zone have a substantially constant depth andsaid first thread has said substantially constant width.
 42. A threadedelement according to claim 1, wherein said at least three threads ofsaid first engaged thread zone have a substantially constant depth, saidgroove is formed in said at least three threads of said first engagedthread zone, and a depth of said groove in said at least three threadsof said substantially constant depth decreases toward said medial threadzone.
 43. A threaded element according to claim 1, wherein said at leastthree threads of said last engaged thread zone have a substantiallyconstant depth, said groove is formed in said at least three threads ofsaid last engaged thread zone, and a depth of said groove in said atleast three threads of said substantially constant depth decreasestoward said medial thread zone.
 44. A threaded element according toclaim 1, wherein all threads have said substantially constant width suchthat said threading is free of any thread that does not have saidsubstantially constant width.